Device operation and information can be gleaned by analyzing the internal address, data, and control bus lines with a logic analyzer, digital oscilloscope, or custom circuitry. Targeted bus lines could be probed by simply removing the soldermask on the circuit board. Be aware of the data being stored in memory at all times and what is transferred across exposed and accessible buses.
Huang's Hacking the Xbox: An Introduction to Reverse Engineering  details the creation of a tap board used to intercept data transfer over the Xbox's HyperTransport bus. Huang was able to retrieve the symmetric encryption key used for protection of a secret boot loader, which ultimately allowed him to execute untrusted code on the system. Moving critical bus lines onto internal layers would thwart such an attack. If a multilayer board is not used, protective encapsulate could at least be applied to the target traces.